HYPOTHESIS: PRIMARY: Augmentation of selective serotonin reuptake inhibitor (SSRI) treatment of Obsessive-Compulsive Disorder w/ondansetron at 3 mg daily for 6 weeks will reduce OCD symptoms significantly more than placebo augmentation. SUBSIDIARY: 1) Augmentation w/ondansetron at 3 mg daily for 11 weeks will reduce OCD symptoms significantly more than augmentation for 6 weeks in the same patients (Shorter vs Longer Treatment). 2) Augmentation w/ondansetron at 6 mg daily for 8 weeks will reduce OCD symptoms significantly more than placebo augmentation for 8 weeks in the same patients (Higher Dose vs Placebo). 3) Augmentation w/ondansetron at 6 mg daily will reduce OCD symptoms significantly more than augmentation at 3 mg daily (Higher Dose vs Lower Dose). 4) Withdrawal of ondansetron treatment at 3 mg daily will be associated w/a statistically significant increase in symptoms (Withdrawal Comparison). SPECIFIC AIMS: 1) Recruit 30 patients w/a DSM-IV diagnosis of OCD into a randomized trial of SSRI plus ondansetron vs SSRI plus placebo. 2) Monitor patient symptoms using weekly Yale Brown Obsessive Compulsive Scale (YBOCS) total score, YBOCS Clinical Global Impression (CGI) and a patient self-rating scales (PSRS, TOUCS). 3) Comparatively analyze treatment conditions for statistically significant changes in YBOCS, patient self-rating scores, and average CGI rating over the course of an 18 week trial.